Investigators said they believe the letter, written on Oct. 22, mailed the next day and delivered to her mother, Zenya Hernandez, on Nov. 6, was written by Hernandez.

"The reason we're announcing this today is that law enforcement had to take very possible step to verify its authenticity, and at this juncture, we believe it was written by Abby and sent to her mother," said Associate Attorney General Jane Young.

The FBI believes the North Conway teen is alive, but in "grave danger."

Timeline: Last known whereabouts of missing girl

Timeline: Last known whereabouts of missing girl

A 14-year-old girl who went missing Oct. 9, 2013, safely returned to her family July 20, 2014 after more than nine months. She turned 15 during her disappearance.

The following is a timeline of the events of her disappearance, eventual return and her alleged captor's arrest.

At 2:29 p.m., the girl was seen leaving Kennett in black leggings, black boots, a dark colored sweater. She walked through the field hockey field, down Eagles Way and into a power line trail near the school.

Between 2:30 and 2:35 p.m. on Wednesday, Hernandez was seen walking on the power line trail near the school.

Hernandez exchanged several text messages with a classmate until 2:53 p.m., which was when her last text was sent.

The FBI said the last known contact between the cell phone and the cellular network was from a tower on the west side of Cranmore Mountain.

The FBI and local authorities later set up several road blocks along North-South Highway to ask drivers if they had seen anything.

Timeline of the last moments the girl was seen.

A week after her disappearance, authorities released a map of the location where they have concentrated their search.

Since her disappearance, authorities have been searching for Abigail for more than a week. Continue through the slideshow to see a timeline of moments from the search.

Crews searched from the ground and air on Friday, Oct. 11.

Between 2:40 and 3:30 p.m., authorities said they have received multiple unconfirmed reports of someone matching the girl's description walking on North-South Highway between the rotary and the entrance to Settlers' Green outlets.

Clear Channel Outdoor donated digital billboard space on Oct. 11 to help the FBI find Abigail Hernandez. Pics of Hernandez will be seen in Mass. and New Hampshire.

In the following days, FBI agents conducted mandatory roadblocks, asking every passing car about the girl and sharing missing posters over the Columbus Day weekend.

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